Deletion Mutants of Francisella Phagosomal Transporters FptA and FptF Are Highly Attenuated for Virulence and Are Protective Against Lethal Intranasal Francisella LVS Challenge in a Murine Model of Respiratory Tularemia

Abstract

Francisella tularensis (Ft) is a Gram-negative, facultative intracellular bacterium that is a Tier 1 Select Agent of concern for biodefense for which there is no licensed vaccine. A subfamily of 9 Francisella phagosomal transporter (fpt) genes belonging to the Major Facilitator Superfamily of transporters was identified as critical to pathogenesis and potential targets for attenuation and vaccine development. We evaluated the attenuation and protective capacity of LVS derivatives with deletions of the fptA and fptF genes in the C57BL/6J mouse model of respiratory tularemia. LVSΔfptA and LVSΔfptF were highly attenuated with LD50 values of >20 times that of LVS when administered intranasally and conferred 100% protection against lethal challenge. Immune responses to the fpt mutant strains in mouse lungs on day 6 post-infection were substantially modified compared to LVS and were associated with reduced organ burdens and reduced pathology. The immune responses to LVSΔfptA and LVSΔfptF were characterized by decreased levels of IL-10 and IL-1β in the BALF versus LVS, and increased numbers of B cells, αβ and γδ T cells, NK cells, and DCs versus LVS. These results support a fundamental requirement for FptA and FptF in the pathogenesis of Ft and the modulation of the host immune response.

Alginate microencapsulation of an attenuated O-antigen mutant of Francisella tularensis LVS as a model for a vaccine delivery vehicle

“…The most effective way to stimulate both arms of the immune system is expected to be through an attenuated strain of F. tularensis that is safe, and adequately protective. Numerous attenuated strains of F. tularensis have been created [14,38,41,42]. However, the efficacy of these strains against high dose respiratory challenge in humans is lacking and safety in immunocompromised patients is questionable.…”

Recent Advancements and Novel Approaches Contributing to the Present Arsenal of Prophylaxis and Treatment Strategies Against Category A Bacterial Biothreat Agents

“…Pathogenicity Determinant Protein C gene deletion mutants imparted high cellular and humoral immune response in Cynomolgus macaques [ 34 ]. Similarly, Francisella phagosomal transporter ( fpt ) mutants of fptA and fptF genes provide defence against lethal doses of Francisella and displayed reduced pathology in mouse model [ 35 ]. Conlan et al [ 36 ] screened putative genes responsible for virulence in Francisella and narrowed down their findings to a clpB gene mutant that outperformed LVS vaccine in intradermal and aerosol challenges with virulent F. tularensis strains in mice.…”